UNIT 1 Force, Motion, and Energy PDF

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UNIT 1 Force, Motion, and Energy 2 UNIT 1: Force, Motion, and Energy Overview In Grade 7, students learned that energy exists in different forms and it can be transformed from one form to another. They also learned that energy can be transferred from one object or place to another in different ways....

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UNIT 1 Force, Motion, and Energy

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UNIT 1: Force, Motion, and Energy Overview In Grade 7, students learned that energy exists in different forms and it can be transformed from one form to another. They also learned that energy can be transferred from one object or place to another in different ways. In Grade 8, students deepen their understanding of the different forms of energy by describing how the energy transferred affects, or is affected by, objects. This unit has six modules. The first two modules discuss the effects of energy at the macroscopic level while the next four modules tackle these effects at the particle level. Module 1 focuses on the idea that if a net or unbalanced force acts on an object, the motion of the object will change. Module 2 picks up this idea and explains how the application of force can do work on an object with a corresponding transfer of energy. Module 3 describes the effects of heat on objects involved in energy transfer and explains these effects at the particle level. Module 4 deals with how energy affects the movement of charges in electrical circuits. Module 5 discusses how energy propagates through solids, liquids, and gases. It also describes how the speed of the energy transferred varies with some factors, such as temperature. Module 6 describes how the different colors of light differ in terms of their frequency and energy. Most of the topics in this module are dealt with qualitatively in order for students to have a basic understanding of the concepts. Some tasks include measurements and computations in order to illustrate the relationship among quantities. Through the activities included in each module, it is also aimed to make students gain interest in these topics and motivate them to learn more in the succeeding grade levels. The following ideas are expected to be developed among the students: 

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Energy is transmitted in the form of heat from one place to another due to temperature differences or in the form of mechanical work (potential and kinetic energy). Energy affects objects. The effects are manifested in the changes that objects undergo. For example, energy can cause changes in the motion of objects, particles, or charges. It can also cause changes in some properties of matter such as temperature. The amount of change depends on the amount of energy transferred. The energy transferred can also be affected by the nature or kind of materials involved.

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Unit 1 MODULE

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FORCES AND MOTION

After learning about the ways by which the motion of an object can be described and represented in grade 7, students will now study the motion of objects using the concept of force. They will describe the effects of forces on an object and determine the relationship between the net force acting on an object and its acceleration due to this force.

Key questions for this module Do forces always result in motion? What are the conditions for an object to stay at rest, to keep moving at constant velocity, or to move with increasing velocity? How is force related to acceleration?

This module aims to address the following misconceptions related to force and motion: 1. If an object stays at rest, there is no force acting upon it. 2. An object continues to move at constant velocity because a constant force acts on it. 3. If the speed of an object increases, its acceleration also increases. 4. Objects move because they have a force; they stop when their force is already used up. (Force is thought to be a property of a material)

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Start the module by eliciting students’ prior knowledge of force and motion. Questions such as the following may be asked:   

What makes objects move the way they do? Why do objects move in different ways? Why are some objects faster than the others? What makes objects stay in place?

Note that there are no correct or wrong answers yet at this point. Just take note of their answers and go back to some of them after they finish the module.

Balanced and Unbalanced Forces 1. To introduce the concept of FORCE, place a ball or any object on top of a table and ask: a) b) c) d) e)

Will this object move by itself? How can we make this object move? While it is moving, how can we make the object speed up or slow down? How can make it stop? How can we make it change its direction?

Ask students to describe or demonstrate how they can achieve the given conditions above. This will lead them to realize that to make the object move, speed up, slow down, stop, or change its direction, it has to be pushed or pulled. The motion of an object can be changed if we apply FORCE on it 2. Pose another question: Consider again this ball here on top of the table. Since this ball stays at rest (meaning it does not change its motion) can we say that there is/are no force/s acting on it?

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Activity

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Forces on objects at rest

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In this activity, students are asked to identify the forces acting on objects at rest. This is a very simple activity and the materials are readily available, so students can work on it individually or in pairs. This is to ensure that everybody is participating during the activity proper.

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At this point, students need not to explain why the objects stay in place. They may explain this after they finish doing Activity 2.

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During the post activity discussion, students can be asked to recall what they learned in the previous grades about the force of gravity. They may be asked to show or cite examples that demonstrate the presence of the force of gravity on Earth. If there is enough time, discuss more about gravitational force. Emphasis should be given on the following ideas: -

Gravitational force is the attraction between any two bodies with mass.

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Gravitational force increases with mass. If the mass of either object increases, the gravitational force between them also increases.

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As the Earth attracts objects around it, these objects also attract the Earth. But the Earth is much more massive than them that is why their attraction is not as great as the gravitational pull of the Earth.

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All things on Earth fall (or are attracted) towards the center of the Earth.

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Answers to Questions

Tension force

Situation 1: Hanging pen Q1.

The pen is at rest.

Q2.

Yes. The forces acting on the pen are the tension force (the force exerted by the string on the pen) and the force of gravity.

Force of gravity

Hanging pen

Q3.

When the string was cut, the pen falls to the ground. The force of gravity makes the object fall down.

Normal force / Force exerted by the table on the book

Situation 2: Book on a table Q4.

The book is at rest.

Q5.

Yes. The forces acting on the book are the force exerted by the table on the book and the force of gravity.

Q6.

No, the book stays at rest. The book may be moved by pushing it on one side only.

Force of gravity

Book on a table

Activity

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Balance of forces

The aim of this activity is to help the students understand how the forces acting on the objects in Activity 1 prevent them from moving. 

In case the number of spring balance is not enough, each group can work on the first part of the activity first using two spring balances. Then they can be asked to

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join with another group to complete the 4 spring balances needed for the four holes around the board. 

During the post activity discussion, the students must realize that there are still forces acting upon objects at rest. But these forces balance each other thereby causing the objects to stay in place. Emphasize the following ideas: -

If two forces acting on an object are equal in magnitude but opposite in direction, they are considered as balanced forces. These forces must lie along the same line.

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If the forces acting on an object are balanced, the object either stays at rest or continues to move at constant velocity.

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If the forces acting on an object are unbalanced, the motion of the object will change. This concept was discussed in the module using the rolling ball as an example. Emphasize that the ball slowed down and eventually stopped not because its force was already used up nor the force acting on it was continuously decreasing (misconceptions). The ball slowed down and stopped because an unbalanced force caused it to change its motion. That unbalanced force is friction. This can be reiterated when Newton’s First Law of Motion is discussed.

Answers to Questions Q7.

The forces are equal in magnitude but opposite in direction.

Q8.

If the lines of action of the forces are extended, they meet at a single point.

Note: At this point, the term “concurrent forces” may be introduced. When the lines of action of the forces acting on an object meet at a single point, they are considered as concurrent forces. When the forces acting on an object are concurrent, the object does not move nor rotate.

Concept check: 1. 2. 3.

Fnet = 20 units Fnet = 5 units. The object will move in the direction of the 10-unit force (larger force). Fnet = 0. The object will not move.

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Newton’s Three Laws of Motion 

If needed, introduce first Isaac Newton to the class. Discuss briefly some of his significant contributions especially in the field of physics. e.g. Newton combined his idea and the ideas of the other scientists like Galileo to give us a more unified picture of how our universe works. He formulated the laws of motion and gravitation. Through his three laws of motion, we can describe and predict the movement of everything around us.

Activity

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Investigating inertia

This activity demonstrates how the inertia of an object affects its motion. Inertia is the tendency of the body to resist changes in its state of motion. This is described through Newton’s First Law of Motion, also referred to as Law of Inertia.

Teaching Tips 

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After discussing the result of the activity, relate the Law of Inertia to the previous discussion on balanced and unbalanced forces. Emphasize that if an object is acted upon by balanced forces, its motion or its velocity will not change. Since acceleration is defined as the change in velocity over time, then we can say that the object will not accelerate. It will only accelerate if the forces acting on it are unbalanced. This is what the Law of Inertia is all about. It states that, “An object will stay at rest or move at constant velocity unless an unbalanced external force acts on it.” If time permits, discuss also the effect of mass on inertia: the greater the body’s mass, the greater will be its inertia. For the application part, relate the concept of inertia to students’ experiences while riding a vehicle. Then discuss the importance of using a seatbelt.

Answers to Questions Coin Drop Q9.

When we slowly pulled the cardboard, the coin on top moved with the cardboard.

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The frictional force acting between the coin and the cardboard caused the coin to stay on top of the cardboard and move with it. Q10.

When the coin was flipped quickly, the cardboard moved forward but the coin did not move with it. When the cardboard was removed from underneath it, the coin dropped into the glass. The coin did not move forward with the coin because of the tendency of the coin to stay at rest (inertia).

Stack of Coins Q11.

When we hit the bottom coin with the edge of the ruler, it moved out from the pile of coins but the other coins stayed in place. The inertia of the coins has caused them not to move out with the coin that was hit by the ruler.

Activity

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Force and acceleration

In this activity, students will describe the relationship between the unbalanced external force acting on an object and its acceleration by analyzing tape charts. 

If the materials are available, try to demonstrate how the data or tape charts were obtained. Hang four identical rubber bands from one end of a wooden bar as shown in Fig. 1. Then mark on the wooden bar the position where the rubber bands should be stretched (Fig. 2). When the rubber band is stretched, it pulls with it the cart. Make sure that the person holding the wooden board with rubber bands is free to move and ready to run, if needed to maintain the length by which the rubber band is stretched while pulling the cart. This is to ensure that the force acting on the cart is constant. The number of rubber bands used to pull the cart is related to the amount of force acting on the cart. If the number of rubber bands is changed, say doubled, the force acting on the cart is considered also to be doubled.

Figure 1 Figure 2

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Since they do not need to perform the activity, students can be asked to work on the tape charts (Figure 3) individually or in pairs. Note that their measurements may differ even if they are provided with the same copies of the tape charts. This is why they are asked to compute for the acceleration of the cart at least three times using different values of average velocity. Then they will just get the average.

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Relate Newton’s Second Law of Motion, also called Law of Acceleration, to the previously discussed topics, particularly on the effects of unbalanced forces on the motion of objects.

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Since the law of acceleration quantifies the relationship among mass, force, and acceleration, it is but necessary to discuss also the effect of mass of the object on its acceleration. As the mass of the object increases, with the same amount of force applied, its acceleration also increases. To state in another way, if the same force acts on two bodies of different masses, the acceleration of the body with lesser mass is greater than the acceleration of the body with greater mass.

Answers to Questions Tape chart analysis Q12.

We noticed that the lengths of the strips in all the tape charts are in increasing order. In terms of the difference, we noticed that the amount of change in length of the strips differs among the tape charts. It is greatest in F=4 units.

Q13.

The increase in lengths of the strips suggests that the average velocity of the cart at equal time interval increases. The cart is accelerating. This is also true to all other tape charts.

Q14.

The increase in length of each strip from one strip to another is of equal size. This indicates equal changes in the velocity of the cart at equal periods of time when the force acting on it is constant. Yes, this is also true with the other tape charts.

Q15.

The increase in length of the strips varies among the four tape charts. The amount of change increases as the units of force increases. The increase in length is greatest in F = 4 units and least in F = 1 unit.

Q16.

When the dots on top of the strips are connected, a straight line was formed. Yes, the same pattern exists for the other tape charts.

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Quantitative analysis Q17.

The computed values of vave are increasing. The cart is accelerating.

Q18.

The computed values of ∆v are equal (or almost equal or very close). This means that the cart is accelerating uniformly or its acceleration is constant.

Q19.

The computed values of acceleration are equal (or almost equal).

Q20.

The acceleration of the cart increases with the net or unbalanced force applied on it. Or as the amount of force applied on the cart increases, the acceleration of the cart also increases.

Activity

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Action-reaction

The Newton's third law of motion, or sometimes called as Law of ActionReaction, describes the relationship between the forces that two bodies exert on each other. In this activity, students should realize that these forces are equal in magnitude but opposite in direction. 

Make clear the difference between this pair of forces and the previously discussed balanced forces. Emphasize that this pair of forces are acting on different bodies, so they do not cancel each other out.

Answers to Questions Q21.

(answer may differ, but the values should be equal) These values represent the amount of pulling force that we exerted on each other.

Q22.

The forces that we exerted are in opposite directions.

Q23.

(The readings this time should be greater than the previous ones)

Q24.

We increased the force that we exerted on each other.

Q25.

(readings may vary)

Q26.

The forces are of opposite directions.

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Figure 3: Tape charts

14 F=1 unit

F= 2 units

F= 3 units

F= 4 units

Solutions: For F = 1 unit

For F = 2 units

V1 = 2.5cm/0.10s = 25 cm/s

V1 = 4.5cm/0.10s = 45 cm/s

V2 = 3.0cm/0.10s = 30cm/s

V2 = 5.5cm/0.10s = 55cm/s

V3 = 3.5cm/0.10s = 35cm/s

V3 = 6.5cm/0.10s = 65cm/s

V4 = 4.0cm/0.10s = 40cm/s

V4 = 7.5cm/0.10s = 75cm/s

V5= 4.5cm/0.10s = 45cm/s

V5= 8.5cm/0.10s = 85cm/s

Solving for a

Solving for a

2 v 2  v 1 30cm/s  25cm/s   50cm/s t 0.10s v 3  v 2 35cm/s  30cm/s 2 a2    50cm/s t 0.10s

2 v 2  v 1 55cm/s  45cm/s   100cm/s t 0.10s v 3  v 2 65cm/s  55cm/s 2 a2    100cm/s t 0.10s

aave  50cm/s 2

a ave  100cm/s 2

For F = 3 units

For F = 4 units

V1 = 8.5cm/0.10s = 85 cm/s

V1 = 14.5cm/0.10s = 145 cm/s

V2 = 10cm/0.10s = 100cm/s

V2 = 16.5cm/0.10s = 165cm/s

V3 = 11.5cm/0.10s = 115cm/s

V3 = 18.5cm/0.10s = 185cm/s

V4 = 13cm/0.10s = 130cm/s

V4 = 20.5cm/0.10s = 205cm/s

V5= 14.5cm/0.10s = 145cm/s

V5= 22.5cm/0.10s = 225cm/s

Solving for a

Solving for a

a1 

a1 

a2 

v 2  v1 t



a1 

2 v  v 1 165cm/s  145cm/s 100cm/s  85cm/s   200cm/s  150cm/sa21  2 t 0.10s 0.10s

v  v 2 185cm/s  165cm/s 2 2 v 3  v 2 115cm/s  100cm/s   200cm/s   150cm/s a 2  3 t 0.10s t 0.10s a ave  200cm/s 2

aave  150cm/s2

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Data for Table 1 Force F = 1 unit F = 2 units F = 3 units F = 4 units

# of rubber bands 1 2 3 4

Acceleration 50 m/s2 100 m/s2 150 m/s2 200 m/s2

Acceleration

200 0 150 0 100 0 50

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2

3 4 Force

Figure 4: Graph of force vs acceleration

References UP NISMED. (2002). Practical Work on High School Physics: Sourcebook for Teachers. UP NISMED. Quezon City

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Unit 1 MODULE

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WORK AND ENERGY

In this module, students will learn about motion from the perspective of work and energy. The concept of energy is one of t...